Heavy-Ion-Induced Electronic Desorption of Gas from Metals

During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss ($d{E}_e/dx$) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering.

Figure 1

Potassium ion-energy loss in stainless steel calculated using the SRIM code, the nuclear ($d{E}_n/dx$), electronic ($d{E}_e/dx$), and total energy losses are displayed.

Figure 2

Effective desorption yields of potassium ions bombarding unbaked stainless steel at various energies and impact angles. For normal incidence, $\theta =0°$. Random variations of $\pm 300$ are indicated by shot-to-shot scatter.

Figure 3

Nuclear ($d{E}_n/dx$) and electronic ($d{E}_e/dx$) energy loss for potassium ions in stainless steel calculated with the SRIM code [1], the upper line is the sum of both. The ion desorption yields divided by 10 000 were measured for impact angles ($\theta =84.0°–85.5°$) and are plotted as the points labeled $\exp ./10000$. The desorption yield error bars ($\pm 300$) take into account the observed fluctuations in ion beam current and pressure rise which were similar for all low-energy range experiments.

Figure 4

Desorption yields measured for 15, 40, and $100\mathrm{MeV}/\mathrm{u}$ ${\mathrm{U}}^{73+}$ ions impacting perpendicularly onto stainless steel. For comparison, the evolution of the electronic energy loss calculated with SRIM [1] is also shown.

Figure 5

Measured ion-induced desorption yields versus electronic energy loss of the corresponding ion (potassium and uranium); square points: data for ${\mathrm{K}}^{+}$ ions with energies between 70 and 1000 keV impacting ($\theta =84.0°–85.5°$) onto stainless steel; diamond points: SIS 18 data for ${\mathrm{U}}^{73+}$ ions with energies 15, 40, and $100\mathrm{MeV}/\mathrm{u}$ impacting perpendicularly ($\theta =0°$) onto stainless steel. The dashed curves indicate the power law ($n$ value) for the electronic component of $dE/dx$.